1. Field of the Invention
This invention generally relates to a valve timing control apparatus for an internal combustion engine. This invention particularly relates to an apparatus for controlling the opening and closing timings of at least one of an inlet valve and an outlet valve in each engine cylinder in response to an operating condition of an internal combustion engine.
2. Description of the Related Art
A known valve timing control apparatus for an internal combustion engine includes a variable valve timing control mechanism which has a hydraulically-controllable cam pulley connected to a camshaft. The cam pulley can vary a cam phase difference relative to crank angle. The variation in the cam phase difference causes a variation in the opening and closing timings of, for example, an inlet valve of each engine cylinder. Generally, the valve timing is adjusted in response to the rotational speed of the engine or the load on the engine.
Regarding the variable valve timing control mechanism, it is known to provide a system for diagnosing a trouble in its operation by referring to the pressure of used hydraulic fluid. It is difficult for such a diagnosis system to detect a trouble which does not cause any abnormality in the hydraulic pressure.
Japanese patent 2590384 discloses a variable valve timing control mechanism provided with a system which diagnoses a trouble in its operation by referring to the phase difference between an engine crankshaft and an engine camshaft.
In a variable valve timing control mechanism of the hydraulic type, if air enters the hydraulic working fluid, the pressure of the fluid tends to be insufficient due to an easy variation in volume of the air. The insufficient hydraulic pressure impairs operation of the variable valve timing control mechanism. In addition, such an insufficient hydraulic pressure may be diagnosed as a trouble in the variable valve timing control mechanism.